As Italy swelters in temperatures of up to 40C, the country's marinas should be packed with bronzed sailors in expensive deck shoes tending to their gleaming yachts.

But Italian boat owners are feeling the heat not just from the sun this summer.

Thousands are weighing anchor and fleeing with their gin palaces to quiet corners of the Mediterranean to escape a tax evasion crackdown – part of efforts by the government of Mario Monti, the prime minister, to tackle Italy's €1.9 trillion public debt.

Six months after the authorities launched a high-profile campaign against suspected tax dodgers in the up-market ski resorts of the Alps and the Dolomites, the battle against evasion has taken to the waves.

Just as tax inspectors targeted the owners of Lamborghinis, Ferraris and other top-of-the-range sports cars in their bid to sniff out tax evasion in the mountains, so in the ports and marinas they are going after the owners of luxury yachts.

In a country where tax evasion is regarded almost as a national sport, they often find that the owners of expensive boats have declared trifling amounts of income, or none at all.

The unwelcome attention has led many yacht owners to flee Italy's marinas for friendlier foreign ports, from Corsica and the Cote d'Azur in the west to Croatia, Slovenia, Montenegro and Greece in the east.

Others are heading southwards, to Malta and Tunisia - where they can access their boats on low-cost budget flights from Italy for a fraction of the tax bill they might otherwise face.

Around 30,000 yachts have fled Italy this year, costing €200 million in lost revenue from mooring fees, port services and fuel sales, according to Assomarinas, the Italian Association of Marinas.

"We've lost 10 to 15 per cent of our regular customers," said Roberto Perocchio, the president of Assomarinas. "This is the worst crisis in Italian boating history. The authorities are using scare tactics and creating a climate of fear."

He said the owners of one yacht in Rapallo, on the Italian Riviera, had put a notice on deck "Have mercy. This boat has already been checked seven times."

The downturn in the industry, coupled with Europe's recession, has cast a dark cloud over the 18,000 new yacht berths now under construction around Italy. Plans for a further 30,000 new berths have been put on hold.

Business is down by more than a third in many marinas, with some half empty compared to last summer.

"We've lost 40 boats in the last few months, all between 20 and 25 metres long," said Giovanni Sorci, director of a marina at Rimini, on the Adriatic coast.

"Most went to Slovenia – in fact it is so popular that there's now barely a berth to be had there.

"It's absolutely right that tax evaders should be caught, but just because you own a boat doesn't mean you are dodging taxes. The authorities are obsessive. They don't just check you once – they come back the next day too.

"You have the police, the Carabinieri, the Guardia di Finanza, the Coast Guard... even the Forestry Corps are involved! No wonder people are leaving."

The one yacht over 25 metres that remains in his marina is there only because tax police impounded it.

Yacht owners are either selling their boats, trading them in for smaller vessels which are a lesser target for tax authorities, or storing them in boat yards, away from prying eyes.

Tax police who raided a marina in the Adriatic port of Bari in April found tax evasion on a breath-taking scale - including a €1.2 million (£1m) yacht whose owner had never filed a tax return.

Another vessel, worth an estimated €700,000 (£580,000), was the property of a company with declared annual revenue of just €1,300.

Of 963 yachts in Bari's picturesque harbour, Guardia di Finanza officers found that 286 had owners with suspiciously low tax returns, or none at all.

At Porto Rotondo in Sardinia, Giacomo Pileri, the general manager of a 700-berth marina, said at least 150 boats had fled to nearby Corsica.

The war on yachting tax evasion gathered pace under Mr Monti's government, which replaced Silvio Berlusconi's discredited coalition in November, but began in 2010. In May that year, the Guardia di Finanza boarded a £17 million mega-yacht owned by Flavio Briatore, the Formula One tycoon, and seized it on suspicion of tax evasion.

Briatore was exonerated and no arrest arrest was made but the raid was a rude shock for Mr Briatore and his wife Elisabetta Gregoraci, a former Wonderbra model, who was on board the 62 metre-long Force Blue with their two-month-old son.

Because the yacht was supposed to be available to charter for €245,000 a week, it was attracted generous tax breaks, but investigators suspected that it was used by Mr Briatore as a floating residence and to throw parties.

Police video footage showing officers approaching the yacht in high-speed patrol boats before boarding it shocked the wealthy yachting community and led to fears that foreign charter companies and their clients might encounter a similar reception.

"People don't know what the implications are for big yachts so they're choosing not to go to Italy or keep their boats there," said Cecile Gauert, editor-in-chief of the US-based Yachts International Magazine. "Boat owners will simply sail to the next friendly harbour if there's something they don't like."

A steep new tax of up to €700 per day on the largest yachts mooring in Italian ports, introduced by the Monti government in December, was watered down in March to exclude foreign-owned boats. But it has further fuelled the exodus of Italian boats abroad.

To avoid the ignominy of an EU bailout, Italy must crack down on tax evasion and the black economy, worth €275 billion a year, or 17.5 per cent of GDP.

But yacht owners have a unique advantage over shops, restaurants and other businesses: they can simply sail their assets away on the open sea.